Successfully complete the game on the hard difficulty setting and get at least a "Regular" rank overall to get the best ending. This includes an extra conversation between Vanessa and "The Client", with Vanessa apparently in the shower. You also will get a new "Game Over" screen.
Successfully complete the game on the normal difficulty setting to get an extra conversation between Vanessa and "The Client"; a new end screen saying "Game over...", and a new title menu.
Successfully complete the game on the easy or normal difficulty setting to unlock the Blackbird suit. It is maxed out on every level and has three super moves from the start. You cannot change those moves or upgrade the suit. You also get a new title menu when you unlock this suit.
Play the trial missions as many times as desired to get extra money to upgrade your suits.
Successfully complete the game on the easy or normal difficulty setting to unlock hard mode.
Successfully complete the game a second time on the same saved game file to unlock the Papillon Suit. This suit is also maxed out but has no barrier; one hit and you are dead. However, you will get to see a lot more of Vanessa. You can also now assign what super moves you want your character to have at each portal and in the inter-mission menus. When you unlock this suit you also unlock a new title menu.